Nanoporous ZnO Photoelectrode for Dye-Sensitized Solar Cell

Nanoporous and macroporous structures were prepared by using self-assembled monolayer (SAM) onto ZnO thin films in order to investigate the efficiency of dye-sensitized solar cells (DSSCs) produced using these films. Using SAM on ZnO thin films, it is obtained successfully assembled large-area, highly ordered porous ZnO thin films. Varying nanoporous radius is observed between 20 and 50 nm sizes, while it is 500–800 nm for macroporous radius. The solar conversion efficiency of 2.75% and IPCE of 29% was obtained using ZnO nanoporous/N719 dye/I−/I3 electrolyte, while macroporous ZnO given solar conversion efficiency of 2.22% and IPCE of 18%.


Introduction
The fundamental properties of II-VI compound semiconductor ZnO have been studied for many years, owing to its direct wide band gap (3.37 eV), large exciton binding energy, huge magnetooptic effect, chemical sensing, piezoelectric and ferroelectric properties, low toxicity, high infrared reflectivity, acoustic characteristics, high electrochemical stability, and excellent electronic properties [1][2][3].Moreover, in the past few years, interesting much research has been reported connected with ZnO nanoscale structures with various morphologies such as nanoporous, nanowires, nanorods, and nanotubes obtained by different methods [4,5].Among these, porous ZnO nanostructures have some specific advantages such as a high specific surface area, chemical and photochemical stability, uniformity in pore size, shapeselective and rich surface chemistry [6].According to the variation of porosity, physical and chemical properties of the samples can be controlled and changed.This makes porous structures a promising material in the field of separation, sensors, catalysis, bioscience, surface acoustic wave device, life science, photonic crystals, light emitting diodes, lasers application, and dye-sensitized solar cells (DSSCs) [7].Since DSSCs based on nanostructure materials offer very low cost and relatively efficient photovoltaic energy conversion, it has attracted much attention in the last decade [8][9][10][11][12].The highest efficiency, to the best of our knowledge, reported on DSSCs was 11% obtained on nanoporous TiO 2 by using ruthenium complex dye, containing I − /I − 3 redox couple electrolytes, and platinum counterelectrode [13][14][15].On the other hand, it has been achieved 0.3-6% conversion efficiency in ZnO nanosemiconductor materials [16,17].It is still thought of as an alternative to TiO 2 because of its ease of crystallization and optical and structural quality.
Reports are available in the preparation of porous ZnO films by a variety of techniques including wet-chemical method, radio frequency magnetron sputtering deposition, electrochemical anodisation, etching surface by an organic and inorganic solvents, templating methods, and selfassembled monolayers (SAMs) which are ordered molecular assemblies of single layer formed via the adsorption of an active surfactant onto the surfaces [18][19][20][21][22].One of the most common examples to the SAMs is alkene thiols such as hexanethiols and dodecanethiols.The SAMs of thiols have been studied widely in recent years, since they offer a rational and easy approach for fabricating interfaces with a welldesigned composition, structural properties and thickness of the samples [23][24][25].
In this study, macro-and nanoporous ZnO structures were formed by using SAMs and DSSC performance is compared to each other in terms of efficiency.Detailed structural and optical characterizations were performed by using scanning electron microscopy (SEM), X-ray diffraction

Experimental
The ECD of ZnO thin films were performed on indium tin oxide (ITO) substrate in a solution of 0.05 M Zn(ClO 4 ) 2 and 0.1 M LiClO 4 into DMSO solvent.ITO substrates were cleaned in an ultrasonic bath with trichloroethylene, acetone, and methanol.A conventional three-electrode ECD system was used for deposition of ZnO thin films.ITO substrate is used as the working electrodes, Ag/AgCl as the reference electrode, and zinc plate as counter electrode.The growth temperature and deposition time were arranged at 100 • C and 1 h, respectively.Deposition potential was kept constant during the growth process as −1.1 V which is determined from cyclic voltammogram (figure not shown).In order to obtain porous ZnO thin films, 0.05 M C 12 SH-SAM and 0.05 M C 6 SH-SAM were used with ethanol solution.After the growth of ZnO thin films, thin films were immersed in these solutions.
ZnO nano-and macroporous structure-based DSSCs were prepared by adsorbing cis-bis(isothiocyanato) bis(2,20bipyridyl-4,40-dicarboxylato)-ruthenium (II) bis tetra butylammonium (N719) dye onto the surfaces of both kinds of samples.Substrates were first heated to 100 • C before immersing them into 0.5 mM ethanolic dye solution for 30 min.The counterelectrode was an Pt : F : SnO 2 substrate.The electrodes were separated by 20 mm polypropylene spacer and pressed together with binder clips.Electrolyte was introduced between the electrodes by capillary forces.The electrolyte consisted of 0.5 M tetrabutylammonium iodide, 0.05 M I 2 (Iodine), and 0.5 M 4-tertbutylpyridine in acetonitrile.Active electrode area was typically 0.25 cm 2 .
The morphology of the nanostructures was analyzed using a JEOL-6400 SEM.The crystal structure was analyzed by XRD (Rigaku D/Max-IIIC diffractometer) with Cu-Kα radiation of 1.54 Å, within the 2θ angle ranging from 20 to 80.The PL measurements were conducted with the RF 5301 PC Shimadzu spectrofluorometer at room temperature.The absorption and transmittance measurements were carried out by Perkin-Elmer UV-VIS Lambda 2S spectrometer.The Raman scattering measurements were performed using a micro-Raman Renishhaw 2000 system with an excitation source of 514.5 nm at room temperature.Current-voltage (I-V ) characteristics were recorded using a Keithley 175A digital multimeter using 0.01 V/s voltage ramp rate.The light source was a 250 W tungsten halogen lamp calibrated to 100 mW/cm 2 using a radiometer (IL1700, International).The absorbance and incident photon to current conversion efficiency (IPCE) were measured using a 1000 W Xe lamp and a monochromator.

Formation Mechanism of the Porous ZnO Structures by
SAMs of C 6 SH and C 12 SH.SAMs are formed by immersing an appropriate substrate into a solution of an organic compound (surface-active material), possessing the ability to spontaneously form an ordered molecular layer on the substrate [26].The driving force for the spontaneous formation of the two-dimensional assembly includes chemical bond formation of molecules with the surface and intermolecular interactions [27].In this study, formation of different size of porous structures on ZnO semiconductors by SAMs of C 6 SH and C 12 SH was achieved.In detail, the constituting selfassembling of the C 6 SH and C 12 SH consists of three parts.The first part is the head group.It causes the exothermic process of chemisorption on the surface of the substrate.It has specific affinity for the substrate.Organization of C 6 SH and C 12 SH starts with the interaction between the head and the substrate by means of chemisorption and last to the thermodynamic equilibrium.The second part is the alkyl chain.It is responsible for the intermolecular distance, the molecular orientation, and the degree of order in the film.The third part is a functional group that constitutes the outer surface of the film.The principal driving force for the formation of these films is a specific interaction between the head group and the substrate surface [28].Provided these interactions are strong, SAMs of C 6 SH and C 12 SH form stable films depending on waiting time into the C 6 SH and C 12 SH solution.Porous structures develop on ZnO due to solvent evaporation at room temperature.Ma and Hao indicated macroporous structures on gold using C 12 SH solution due to solvent evaporation and water droplet condensation [29].They showed that the rapid evaporation of the solvent decreases the air/liquid interfacial temperature, resulting in porous formation on substrate.Similarly, Cai et al. showed porous formation via SAMs and indicated that different solvent evaporation rates affect pore formation and pore size [30].
The structural properties of the porous ZnO, which is obtained via SAMs of C 6 SH and C 12 SH, are strongly influenced by the interactions between the functional groups comprising the alkyl chain and functional group of the molecules as it has been shown our previous study [25].The porous formation is largely governed by molecular-substrate interactions, though lateral chain-chain interactions are also quite important.While porous formation on ZnO substrate occurs via SAMs of C 6 SH and C 12 SH, it was shown that films formed by short alkyl chains exhibit very different characteristics than those formed by long alkyl chains.The alkyl chain and functional groups are found to have a profound influence on the porous structure.This knowledge will undoubtedly prove useful in controlling the surface densities of the samples which are prepared via SAMs.

Structural and Optical Properties of the Porous ZnO.
X-ray diffraction measurement of nano-and macroporous ZnO has been shown that the films have highly preferred (0002) c-axis orientation as shown in Figures 1(a) and 1(b), respectively.Another peak, ZnO (0004), can be seen in the Figure 1 porous radius has a negative effect on the crystallization as expected.
Figures 1(c) and 1(d) show SEM images of the porous structures formed onto the ZnO thin films which were obtained by using SAMs technique.Figure 1(c) shows that the nanoporous ZnO thin films have successfully been obtained with porous radius around 50 nm as indicated on image and also uniform pore distribution can be seen through the surface.Average radius of the pores has been calculated by taking account of the pores having a maximum, minimum and average size as indicated in the Figure 1(c).Macroporous structures are shown in Figure 1(b).Perfect circular structures of the pores can be seen with an average pore radius of around ∼800 nm as it is indicated in the figure .PL characterizations of the nano-and macroporous ZnO thin films are shown as an inset of the respective SEM images in Figures 1(c) and 1(d).PL measurements exhibit different spectra for both samples.Two luminescence bands are observed in nanoporous ZnO thin films.One of them is relatively weak deep level emission peaking at the 466 nm, while narrow UV emission is dominant emission peaking at about 362 nm which might be responsible for the recombination of the free excitons.As can be seen in the Figure 1(d), macroporous structure has a strong green emission peak located at ∼530 nm.This peak intensity is much higher compared to the intensity of the UV emission at ∼363 nm confirming that the decrease in the optical quality of the film [23][24][25].Also, it can be speculated that the deep level defect formation causing the green emission in ZnO, which is believed commonly either oxygen vacancy or zinc vacancy [3], is more favorable in macroporous sample than the nanoporous sample.
The UV-visible absorbance and transmittance spectra of the ZnO nanoporous and macroporous are shown in Figures 2(a) and 2(b), respectively.The absorption spectra of the ZnO porous structures showed a strong absorption between 300 nm and 360 nm.The ZnO nanoporous sample has a good transparency with a visible light transmittance higher than 85%.However, the ZnO macroporous structures exhibited low transmittance at around 60%, which might be related to the pore density.These results clearly indicate that transparency of the ZnO nanoporous structure are related to surface morphology and roughness which affects the light scattering.Optical band gap of the ZnO nanoporous structures is calculated by using the following equation [30,31]: where α is absorption coefficient.The direct band gap E g was determined from α 2 -photon energy plot as shown in  Raman spectra for the nanostructures were obtained to investigate the vibration properties of the nano and macroporous ZnO films.Wurtzite ZnO nanostructures belong to the C 4 6v (P6 3 mc) symmetry group [32].At the Γ point of the Brillouin zone, group theory predicts the existence of the following optic modes [33]: (2) A 1 , E 1 , and E 2 modes are Raman active, and B 1 is forbidden.In addition, A 1 , E 1 are infrared active and split into longitudinal (LO) and transverse (TO) optical components.The Raman spectra for the nanostructures in Figure 3 show that the ZnO nanoporous and macroporous structures have the similar Raman shift at the range of 100-900 cm −1 .A dominant and strong intensity peak at 438 cm −1 indicates the spectrum known as the optical phonon E 2 .The E 2 mode corresponds to the band characteristic for the Wurtzite hexagonal phase of ZnO raman active branches, which is also one of the characteristics of ZnO nanostructures [34].However, two short peaks at 331 and 380 cm −1 were shown to be as E 2H -E 2L (multiphonon) and A 1T modes, respectively.For the ZnO nanostructures, an extra Raman band at 582 cm −1 known to be related to the E 1 mode because of the oxygen deficiency [35] indicates the presence of oxygen vacancies in the ZnO nanostructures.
Figure 4 shows the comparison of the current densityvoltage (J-V ) characteristics of solar cells fabricated using the ZnO nano-and macroporous structures, under 100 mW/cm 2 of AM-1.5 illumination.It can be seen from Figure 4 that there is a considerable increase in the shortcircuit current density (J sc ) of the nanoporous ZnO DSSCs in comparison with the macroporous ZnO DSSCs.ZnO nanoporous structures provide both a larger surface area and a direct pathway for electron transport along the pore resulting in the observed higher J sc [36,37].This is attributed to the superior light absorbing characteristics provided by the larger surface areas formed by the nanoporous structure.
The open-circuit voltage (V oc ) of the DSSCs shows that the higher solar conversion efficiency (η) of 2.75% is reached for the nanoporous structures, while it is 2.22% for macroporous structure.A comparison of the different solar cell parameters for the both DSSCs is summarized in Table 1.It can be seen that the ZnO nanoporous structures shows η values that are almost 40% higher than that of the macroporous ZnO.This is a significant result showing the dependence of solar cell performance on surface morphology of the ZnO.Obtained larger surface area in nanoporous structure also results in improved dye adsorption [38][39][40].nanoporous DSSCs show much higher IPCE, 29% at about 510 nm, compared to the macroporous ZnO, 18% at about 520 nm, mainly due to the aforementioned effect of larger surface area (and dye-loading).
Several attempts have been made to use ZnO photoelectrode in DSSCs.Redmond et al. [41] achieved an IPCE of 13% at 520 nm when using the Ru(dcbpyH2) 2 (NCS) 2 dye.On the other hand, Keis et al. introduced IPCE values of 50-60% at 540 nm in photoelectrochemical (PEC) and an overall solar energy conversion efficiency of 2.0% was obtained under 56 mW/cm 2 illumination with a solar simulator [42,43] which is lower than the present study.Recently, solar energy conversion efficiency under 99 mW/cm 2 illumination reaching 2.5% has been reported for then mercurochromesensitized ZnO PEC solar cell [44].However, the efficiency is still moderate compared to solar cells based on TiO 2 .

Conclusion
In this study, using SAMs technique on ZnO thin films, we have successfully formed an assembled large-area, highly ordered porous ZnO thin films.Average pore radius is obtained around 50 nm, and 800 nm for nanoporous and macroporous ZnO structures, respectively.PL measurements show highly different emission characteristics according to the size of the porous radius: the samples having a macroporous show that green emission is the dominant, while, in the nanoporous sample the UV emission is dominant showing to better optical quality of the sample.In addition, the origin of an extra Raman band at 582 cm −1 was obtained due to oxygen deficiency of the ZnO nanostructures.The solarto-electric energy conversion efficiency of 2.75% and IPCE of 29% was obtained by using the ZnO nanoporous/N719 dye/I − /I − 3 electrolyte.In addition, Zn macroporous showed solar-to-electric energy conversion efficiency of 2.22% and IPCE of 18%.In conclusion, ZnO nanoporous electrodes have been studied into dye-sensitized solar cells where they

Figure 1 :
Figure 1: (a) XRD plot of nanoporous ZnO, (b) XRD plot of macroporous ZnO, (c) SEM image and PL characterization of nanoporous ZnO, (d) SEM image and PL characterization of macroporous ZnO.

Figure 2 :
Figure 2: (a) Absorbance.(b) Transmittance spectrum of ZnO nanoporous and macroporous structures at room temperature.(c) Absorption coefficient and energy plot for ZnO nanoporous.(d) Absorption coefficient and energy plot for ZnO macroporous.

Figure 2 (
Figure 2(c) and 2(d).The optical bandgaps were determined as 3.37 eV and 3.42 eV for ZnO nanoporous and macroporous, respectively.Different optical band gap values obtained have shown that optical properties of the ZnO structures can be changed by the change in pore radius formed on the surfaces.Raman spectra for the nanostructures were obtained to investigate the vibration properties of the nano and macroporous ZnO films.Wurtzite ZnO nanostructures belong to the C 4 6v (P6 3 mc) symmetry group[32].At the Γ point of the Brillouin zone, group theory predicts the existence of the following optic modes[33]: shift (cm −1 )

Table 1 :
J-V characteristics of the cell using ZnO nanostructures with N719.